(a) Field of the Invention
The present invention relates to a heat-sensitive transfer medium utilizing a color-forming reaction between a leuco dye and a developer therefor, in more detail relates to a heat-sensitive transfer medium which is capable of producing a high density transfer image by the use of micro-energy and further producing uniform density transfer images even when the transfer operation is repeated a large number of times.
(b) Description of the Prior Art
As the conventional heat-sensitive transfer mediums, there are known the one comprising the combination of a transfer sheet made by providing a thermo-sublimating dye layer on a substrate with a receiving sheet, and the one comprising the combination of a transfer sheet made by providing a transfer layer containing a heat fusible substance such as fatty acid amide and a chromatogenous agent (dye or pigment) on a substrate with a receiving sheet. When actually forming images by the use of the heat-sensitive transfer medium of this type is carried out in the manner of superposing a receiving sheet on the surface of a heat fusible dye layer or transfer layer of a transfer sheet and heat-pressing with a printing means such as a thermal head, a thermal pen or the like from the back surface of said transfer sheet (the surface of said substrate) to thereby sublimation transfer said dye imagewise on said receiving sheet or fusion transfer the transfer layer imagewise on the receiving sheet.
However, the former heat-sensitive transfer medium using the thermo-sublimating dye in the receiving sheet is defective in that the dye image formed on the receiving sheet is inferior in preservability and therefore an over coat must be applied on the transfer image, while the latter heat-sensitive transfer medium using the transfer sheet having the transfer layer formed by dispersing the chromatogenous agent in the heat fusible substance is defective in that when a large amount of chromatogenous agent is incorporated in the transfer layer in order to obtain high density images, the efficiency of transfer deteriorates, and consequently it is difficult to obtain high density images, and further when a large amount of heat fusible substance is used in the transfer layer, said large amount of heat fusible substance transfer to the receiving sheet side, and consequently when stripping the transfer sheet and the receiving sheet, they are not stripped smoothly, the fine-lined up image area becomes indistinct and the like.
On the other hand, there is also known the heat-sensitive transfer medium wherein two kinds of substances such, for instance, as a leuco dye and a developer like a phenolic substance which thermally react each other to bring out the color, are carried on a separate substrate respectively by the use of aforesaid heat fusible substance to make-up a transfer sheet and a receiving sheet respectively, the transfer layer of the transfer sheet (the layer having carried a leuco dye thereon) and the receiving layer of the receiving sheet (the layer having carried a developer thereon) are superposed so that said both sheets may surface-contact, and thermoprinting is carried out from the back surface of the transfer sheet. However, the heat-sensitive transfer medium of this type can be observed defective in that due to its reactive type, a sufficient color-forming reaction can not be carried out in case the transfer layer is merely transferred to the receiving layer at the time when the surfaces of both layers contact, so that the low density images may be produced, and that in case the thermoprinting is carried out for a long period of time under the elevated heating condition, the image on the receiving sheet becomes more high density, but on the other hand the color-forming reaction progresses also on the transfer sheet thereby to form images.